Inserting a single-cell table with top border is a common workaround for the missing 'horizontal line' functionality. 

Exporting such table to PDF causes wrong result if the intercepting vertical borders are of type None and have smaller thickness.

Attached image outlines the issue.

Workaround: Set the same thickness to the noBorder and to the visible border:
var t = new Telerik.Windows.Documents.Model.Table(1, 1);\
var noBorder = new Telerik.Windows.Documents.Model.Border(3, BorderStyle.None, Colors.Black);
t.Borders = new TableBorders(noBorder, new Telerik.Windows.Documents.Model.Border(3, BorderStyle.Single, Colors.Black), noBorder, noBorder);


Steps to reproduce:
1. Insert table with the following code:

            var t = new Telerik.Windows.Documents.Model.Table(1, 1);
            var noBorder = new Telerik.Windows.Documents.Model.Border(BorderStyle.None);
            t.Borders = new TableBorders(noBorder, new Telerik.Windows.Documents.Model.Border(3, BorderStyle.Single, Colors.Black), noBorder, noBorder);

            this.radRichTextBox.InsertTable(t);
2. Export to PDF